Oceanus Procellarum

Oceanus Procellarum (/oʊˈsiːənəsˌprɒsɪˈleɪrəm/; Latin for "Ocean of Storms") is a vast lunar mare on the western edge of the near side of the Moon. It is the only one of the lunar maria to be called an "Oceanus" (ocean), due to its size: Oceanus Procellarum is the largest of the maria, stretching more than 2,500 km (1,600 mi) across its north-south axis and covering roughly 4,000,000 km2 (1,500,000 sq mi), accounting for roughly 10.5% of the total lunar surface area.

Oceanus Procellarum

Oceanus Procellarum is the large mare in the center and upper left of this image.

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Like all lunar maria, Oceanus Procellarum was formed by ancient basaltic flood volcanic eruptions that covered the region in a thick, nearly flat layer of solidified magma. Unlike the other lunar maria, however, Oceanus Procellarum may or may not be contained within a single well-defined impact basin. Around its edges lie many minor bays and seas, including Mare Nubium and Mare Humorum to the south. To the northeast, Oceanus Procellarum is separated from Mare Imbrium by the Carpathian Mountains. On its north-west edge lies the 32 km wide Aristarchus ray crater, which is considered as the brightest feature on the Near side of the Moon.[3] Also, the more-prominent ray-crater Copernicus lies within the eastern edge of the mare, distinctly with its bright ray materials sprawling over the darker material. On the northern edge of Oceanus Procellarum lies Sinus Roris.

There are several hypotheses about the origin of Oceanus Procellarum and a related asymmetry between the near and far sides of the Moon. One of the most likely is that Procellarum was a result of an ancient giant impact on the near side of the Moon. The size of the impact basin has been estimated to be more than 3,000 kilometers, which would make it one of the three largest craters in the Solar System.

The impact likely happened very early in the Moon's history: at the time when magma ocean still existed or just ceased to exist. It deposited 5–30 km of crustal material on the far side forming highlands. If this is the case, all impact related structures such as crater rim, central peak etc. have been obliterated by later impacts and volcanism. One piece of evidence in support of this hypothesis is concentration of incompatible elements (KREEP) and low calcium pyroxene around Oceanus Procellarum.[4][5]

Procellarum may have also been formed by spatially inhomogeneous heating during the Moon's formation.[4] The GRAIL mission, which mapped the gravity gradients of the Moon, found square formations resembling rift valleys surrounding the region beneath the lava plains, suggesting the basin was formed by heating and cooling of the lunar surface by internal processes rather than by an impact, which would have left a round crater.[6]

Other hypotheses include a late accretion of a companion Moon on the far side. The latter postulates that in addition to the present Moon, another smaller (about 1,200 km in diameter) moon was formed from debris of the giant impact. After a few tens of millions of years it collided with the Moon and due to a small collisional velocity simply piled up on one side of the Moon forming what is now known as far side highlands.[7]

During the Apollo program, flight operations planners were concerned about having the optimum lighting conditions at the landing site, hence the alternative target sites moved progressively westward, following the terminator. A delay of two days for weather or equipment reasons would have sent Apollo 11 to Sinus Medii (designated ALS3) instead of ALS2—Mare Tranquillitatis; another two-day delay would have resulted in ALS5, a site in Oceanus Procellarum, being targeted.